A liquid crystal display device uses polarized light. To this end, a polarizing plate is used, and a PVA element is typically used. However, a polarizing plate such as a PVA element has weak mechanical properties and is easily influenced by an external environment such as temperature and humidity. Thus, a protective film for protecting the polarizing plate is required.
Such a protective film should have not only excellent optical properties but also excellent mechanical properties. A TAC film (Tri-Acetyl-cellulose Film) has been commonly used as a protective film of a PVA element used for a polarizing plate, but in recent years, an acrylic film having more excellent heat resistance and absorption resistance properties as compared to the TAC film is used.
The acrylic films for protecting the polarizing plate are prepared through stretching process. The acrylic resin having a glass transition temperature of 120° C. or higher is generally used so that the dimensional change at a high temperature is small and the optical properties are stably maintained. In addition, in order to further improve dimensional stability and optical properties of the acrylic resin, a monomer having a cyclic structure imparting heat resistance is introduced. However, when a monomer having a cyclic structure is introduced, there is a problem that not only the unit price of the raw material is increased but also the processing must be performed at a higher temperature.
On the other hand, although polymethyl methacrylate (PMMA) is excellent in transparency and thus can be used as a polarizing plate protective film, it has a low glass transition temperature, which causes a problem that, during use at a high temperature, a stretching history is loosened and a dimensional stability is deteriorated. In addition, for the purpose of using it as a polarizing plate protective film for IPS mode, another retardation-adjusting agent must be added in order to realize a low retardation value, and the retardation-adjusting agent used herein must have excellent compatibility with polymethyl methacrylate and must be included in an appropriate amount for implementing a low retardation value.
Further, when a film is produced by stretching polymethyl methacrylate, the stretched film has a negative birefringence property in which the refractive index becomes large in a direction perpendicular to the stretching direction. Therefore, the retardation-adjusting agent used for implementing a low retardation value should have a positive birefringence property in which the refractive index in the stretching direction becomes large. Polycarbonate, polyester, phenoxy resin and the like are known as materials having such positive birefringence property, but most of them have a disadvantage that compatibility with polymethyl methacrylate is poor.
Accordingly, the present inventors have made extensive and intensive studies to produce a resin composition for optical material capable of realizing a low retardation value while using polymethyl methacrylate not including a cyclic monomer in a polymer main chain. As a result, it has been found that the resin composition for optical material containing a specific amount of methacrylic acid monomer at the terminal of the polymethyl methacrylate and containing the polycarbonate as a retardation-adjusting agent exhibits the above-mentioned properties, thereby completing the present invention.